diff --git a/lib/missingNumber/index.ts b/lib/missingNumber/index.ts
new file mode 100644
index 0000000..39d30a8
--- /dev/null
+++ b/lib/missingNumber/index.ts
@@ -0,0 +1,3 @@
+import { missingNumber } from "./missingNumber";
+
+export default missingNumber;
diff --git a/lib/missingNumber/missingNumber.test.ts b/lib/missingNumber/missingNumber.test.ts
new file mode 100644
index 0000000..724c5ee
--- /dev/null
+++ b/lib/missingNumber/missingNumber.test.ts
@@ -0,0 +1,31 @@
+import { describe, it, expect } from "vitest";
+import { missingNumber } from "./missingNumber.ts";
+
+describe("missingNumber", () => {
+
+  it("[3, 0, 1] expect 2", () => {
+    const nums = [3, 0, 1];
+    expect(missingNumber(nums)).toBe(2);
+  });
+
+  it("[0, 1] expect 2", () => {
+    const nums = [0, 1];
+    expect(missingNumber(nums)).toBe(2);
+  });
+
+  it("[9, 6, 4, 2, 3, 5, 7 , 0, 1] expect 8", () => {
+    const nums = [9, 6, 4, 2, 3, 5, 7, 0, 1];
+    expect(missingNumber(nums)).toBe(8);
+  });
+
+  it("[0] expect 1", () => {
+    const nums = [0];
+    expect(missingNumber(nums)).toBe(1);
+  });
+
+  it("[1, 2] expect 0", () => {
+    const nums = [1, 2];
+    expect(missingNumber(nums)).toBe(0);
+  });
+
+});
diff --git a/lib/missingNumber/missingNumber.ts b/lib/missingNumber/missingNumber.ts
new file mode 100644
index 0000000..5b01cc2
--- /dev/null
+++ b/lib/missingNumber/missingNumber.ts
@@ -0,0 +1,13 @@
+export function missingNumber(nums: number[]) {
+
+  let overallSum = 0;
+  for (let i = 0; i <= nums.length; i++) {
+    overallSum += i;
+  }
+
+  for (const num of nums) {
+    overallSum -= num;
+  }
+
+  return overallSum;
+};